Biochemistry

The Structure and Function of Macromolecules

Carbon—Backbone of Biological Molecules

• Although cells are 70–95% water, the rest consists mostly of carbon-based compounds

• Carbon is unique in its ability to form large, complex, and diverse molecules

• Proteins, DNA, carbohydrates, and other molecules that distinguish living matter are all composed of carbon compounds

Organic chemistry-the study of carbon compounds

• Organic compounds range from simple molecules to colossal ones

• Most organic compounds contain hydrogen atoms in addition to carbon atoms

• With four valence electrons, carbon can form four covalent bonds with a variety of atoms

• Needs 4 electrons - single, double or triple bonds

• This tetravalence makes large, complex molecules possible - can form long chains or rings

Carbon Molecules• In molecules with multiple carbons, each carbon

bonded to four other atoms has a tetrahedral shape• However, when two carbon atoms are joined by a

double bond, the molecule has a flat shapeMolecularFormula

StructuralFormula

Ball-and-StickModel

Space-FillingModel

Methane

Ethane

Ethene (ethylene)

Carbon Molecules• The electron configuration of carbon gives it covalent

compatibility with many different elements• The valences of carbon and its most frequent partners

(hydrogen, oxygen, and nitrogen) are the “building code” that governs the architecture of living molecules

Hydrogen

(valence = 1)

Oxygen

(valence = 2)

Nitrogen

(valence = 3)

Carbon

(valence = 4)

Carbon Skeleton Diversity

• Carbon is a versatile atom

• Carbon can use its bonds to form an endless diversity of carbon skeletons

• Carbon chains form the skeletons of most organic molecules

LengthEthane Propane

Butane 2-methylpropane(commonly called isobutane)

Branching

Double bonds

Rings

1-Butene 2-Butene

Cyclohexane Benzene

Hydrocarbons

• Hydrocarbons are organic molecules consisting of only carbon and hydrogen

• Many organic molecules, such as fats, have hydrocarbon components

• Hydrocarbons can undergo reactions that release a large amount of energy

Isomers

• Isomers are compounds with the same molecular formula but different structures and properties:– Structural isomers have

different covalent arrangements of their atoms

– Geometric isomers have the same covalent arrangements but differ in spatial arrangements

– Enantiomers are isomers that are mirror images of each other

Structural isomers differ in covalent partners, as shown in this example of two isomers of pentane.

Geometric isomers differ in arrangement about a double bond. In these diagrams, X represents an atom or group of atoms attached to a double-bonded carbon.

cis isomer: The two Xsare on the same side.

trans isomer: The two Xsare on opposite sides.

L isomer D isomer

Enantiomers differ in spatial arrangement around an asymmetric carbon, resulting in molecules that are mirror images, like left and right hands. The two isomers are designated the L and D isomers from the Latin for left and right (levo and dextro). Enantiomers cannot be superimposed on each other.

Enantiomers• Enantiomers are important in the pharmaceutical industry• Two enantiomers of a drug may have different effects• Differing effects of enantiomers demonstrate that organisms

are sensitive to even subtle variations in molecules

L-Dopa(effective againstParkinson’s disease)

D-Dopa(biologicallyInactive)

Functional Groups• Distinctive properties of organic molecules

depend not only on the carbon skeleton but also on the molecular components attached to it

• Certain groups of atoms called functional groups are often attached to skeletons of organic molecules

• Functional groups are the parts of molecules involved in chemical reactions

• The number and arrangement of functional groups give each molecule its unique properties

Functional Groups• The six functional groups that are most important in the

chemistry of life:– Hydroxyl group– Carbonyl group– Carboxyl group– Amino group– Sulfhydryl group– Phosphate group

STRUCTURE

(may be written HO—)

NAME OF COMPOUNDS

Alcohols (their specific names

usually end in -ol)

Ethanol, the alcohol present in

alcoholic beverages

FUNCTIONAL PROPERTIES

Is polar as a result of the

electronegative oxygen atom

drawing electrons toward itself.

Attracts water molecules, helping

dissolve organic compounds such

as sugars (see Figure 5.3).

STRUCTURE

NAME OF COMPOUNDS

Ketones if the carbonyl group is

within a carbon skeleton

EXAMPLE

Acetone, the simplest ketone

A ketone and an aldehyde may

be structural isomers with

different properties, as is the case

for acetone and propanal.

Aldehydes if the carbonyl group is

at the end of the carbon skeleton

Acetone, the simplest ketone

Propanal, an aldehyde

FUNCTIONAL PROPERTIES

STRUCTURE

NAME OF COMPOUNDS

Carboxylic acids, or organic acids

EXAMPLE

Has acidic properties because it isa source of hydrogen ions.

Acetic acid, which gives vinegarits sour taste

FUNCTIONAL PROPERTIES

The covalent bond betweenoxygen and hydrogen is so polarthat hydrogen ions (H+) tend todissociate reversibly; for example,

Acetic acid Acetate ion

In cells, found in the ionic form,which is called a carboxylate group.

STRUCTURE

NAME OF COMPOUNDS

Amine

EXAMPLE

Because it also has a carboxyl

group, glycine is both an amine and

a carboxylic acid; compounds with

both groups are called amino acids.

FUNCTIONAL PROPERTIES

Acts as a base; can pick up a

proton from the surrounding

solution:

(nonionized)

Ionized, with a charge of 1+,under cellular conditions

Glycine

(ionized)

STRUCTURE

(may be written HS—)

NAME OF COMPOUNDS

Thiols

EXAMPLE

Ethanethiol

FUNCTIONAL PROPERTIES

Two sulfhydryl groups can

interact to help stabilize protein

structure (see Figure 5.20).

STRUCTURE

NAME OF COMPOUNDS

Organic phosphates

EXAMPLE

Glycerol phosphate

FUNCTIONAL PROPERTIES

Makes the molecule of which it

is a part an anion (negatively

charged ion).

Can transfer energy between

organic molecules.

Biochemistry: The Molecules of Life

• Within cells, small organic molecules are joined together to form larger molecules

• Macromolecules are large molecules composed of thousands of covalently connected atoms

Biochemistry• Living organisms are composed of both inorganic

and organic molecules• Inorganic molecules

– Relatively small, simple molecules that usually lack C (a few have one C atom).

– Examples: CO2, NH3, H2O, O2, H2

• Organic molecules– Larger, more complex, based on a backbone of C atoms

(always contain C as a major part of their structure). – Examples: C6H12O6, C2H5COOH

Macromolecules - Polymers• A polymer is a long molecule consisting of many similar

building blocks called monomers• Most macromolecules are polymers, built from monomers• An immense variety of polymers can be built from a small set

of monomers• Three of the four classes of life’s organic molecules are

polymers:– Carbohydrates– Proteins– Nucleic acids

Polymers• Monomers form larger

molecules by condensation reactions called dehydration reactions

• Polymers are disassembled to monomers by hydrolysis, a reaction that is essentially the reverse of the dehydration reaction

Short polymer Unlinked monomer

Dehydration removes a watermolecule, forming a new bond

Dehydration reaction in the synthesis of a polymer

Longer polymer

Hydrolysis adds a watermolecule, breaking a bond

Hydrolysis of a polymer

Carbohydrates• Carbohydrates serve as fuel and building material• They include sugars and the polymers of sugars• The simplest carbohydrates are monosaccharides,

or single (simple) sugars• Carbohydrate macromolecules are

polysaccharides, polymers composed of many sugar building blocks

Sugars

• Monosaccharides have molecular formulas that contain C, H, and O in an approximate ratio of 1:2:1

• Monosaccharides are used for short term energy storage, and serve as structural components of larger organic molecules

• Glucose is the most common monosaccharide

• Monosaccharides are classified by location of the carbonyl group and by number of carbons in the carbon skeleton

• 3 C = triose e.g. glyceraldehyde • 4 C = tetrose • 5 C = pentose e.g. ribose, deoxyribose • 6 C = hexose e.g. glucose, fructose, galactose • Monosaccharides in living organisms generally

have 3C, 5C, or 6C:

Monosaccharides

Triose sugars(C3H6O3)

GlyceraldehydeAld

ose

sK

eto

s es

Pentose sugars(C5H10O5)

Ribose

Hexose sugars(C5H12O6)

Glucose Galactose

Dihydroxyacetone

Ribulose

Fructose

Monosaccharides• Monosaccharides serve as a

major fuel for cells and as raw material for building molecules

• The monosaccharides glucose and fructose are isomers– They have the same chemical

formula– Their atoms are arranged

differently

• Though often drawn as a linear skeleton, in aqueous solutions they form rings Glucose Fructose

Monosaccharides

• In aqueous solutions, monosaccharides form rings

Linear andring forms

Abbreviated ringstructure

29

H H H H H

H

OH OH

OH O

OH H

OH O

CH2OH

Ribose Glyceraldehyde

Triose (3-carbon sugar)

Pentoses (5-carbon sugars)

Deoxyribose

H H

H

H

H

H

OH

OH

O C

C

C 4

5

1

3 2

4

5

1

3 2

1

3

2

CH2OH

Disaccharides• A disaccharide is formed when a dehydration reaction joins two

monosaccharides• Disaccharides are joined by the process of dehydration synthesis• This covalent bond is called a glycosidic linkage

Glucose

Maltose

Fructose Sucrose

Glucose Glucose

Dehydrationreaction in thesynthesis of maltose

Dehydrationreaction in thesynthesis of sucrose

1–4glycosidic

linkage

1–2glycosidic

linkage

Disaccharides

• Lactose = Glucose + Galactose• Maltose = Glucose + Glucose• Sucrose = Glucose + Fructose• The most common disaccharide is

sucrose, common table sugar• Sucrose is extracted from sugar cane and

the roots of sugar beets

Polysaccharides• Complex carbohydrates are called polysaccharides

• They are polymers of monosaccharides - long chains of simple sugar units

• Polysaccharides have storage and structural roles

• The structure and function of a polysaccharide are determined by its sugar monomers and the positions of glycosidic linkages

(a) Starch

(b) Glycogen

(c) Cellulose

Storage Polysaccharides - Starch

• Starch, a storage polysaccharide of plants, consists entirely of glucose monomers

• Plants store surplus starch as granules within chloroplasts and other plastids

Chloroplast Starch

1 µm

Amylose

Starch: a plant polysaccharide

Amylopectin

Storage Polysaccharides - Glycogen

• Glycogen is a storage polysaccharide in animals

• Humans and other vertebrates store glycogen mainly in liver and muscle cells

Mitochondria Glycogen granules

0.5 µm

Glycogen

Glycogen: an animal polysaccharide

Structural Polysaccharides• Cellulose is a major

component of the tough wall of plant cells

• Like starch, cellulose is a polymer of glucose, but the glycosidic linkages differ

• The difference is based on two ring forms for glucose: alpha () and beta ()– Polymers with alpha

glucose are helical

– Polymers with beta glucose are straight

a Glucose

a and b glucose ring structures

b Glucose

Starch: 1–4 linkage of a glucose monomers.

Cellulose: 1–4 linkage of b glucose monomers.

Cellulose • Enzymes that digest starch by

hydrolyzing alpha linkages can’t hydrolyze beta linkages in cellulose

• Cellulose in human food passes through the digestive tract as insoluble fiber

• Some microbes use enzymes to digest cellulose

• Many herbivores, from cows to termites, have symbiotic relationships with these microbes

Cellulosemolecules

Cellulose microfibrilsin a plant cell wall

Cell walls Microfibril

Plant cells

0.5 µm

Glucosemonomer

Chitin• Chitin, another structural polysaccharide, is found in the

exoskeleton of arthropods

• Chitin also provides structural support for the cell walls of many fungi

• Chitin can be used as surgical thread

Lipids• Lipids are the one class of large biological molecules

that do not form polymers• Utilized for energy storage, membranes, insulation,

protection• Greasy or oily substances• The unifying feature of lipids is having little or no

affinity for water - insoluble in water • Lipids are hydrophobic becausethey consist mostly

of hydrocarbons, which form nonpolar covalent bonds

Fats• The most biologically important lipids are fats,

phospholipids, and steroids• Fats are constructed from two types of smaller molecules:

glycerol and fatty acids• Glycerol is a three-carbon alcohol with a hydroxyl group

attached to each carbon• A fatty acid consists of a carboxyl group attached to a long

carbon skeleton

Dehydration reaction in the synthesis of a fat

Glycerol

Fatty acid(palmitic acid)

Fatty Acids• A fatty acid has a long hydrocarbon chain with a

carboxyl group at one end.

• Fatty acids vary in length (number of carbons) and in the number and locations of double bonds

• Saturated fatty acids have the maximum number of hydrogen atoms possible and no double bonds

• Unsaturated fatty acids have one or more double bonds, – Monounsaturated (one double bond)– Polyunsaturated (more than one double bond)

• H can be added to unsaturated fatty acids using a process called hydrogenation

• The major function of fats is energy storage

Stearate Oleate

Fats• Fats separate from water because water molecules form

hydrogen bonds with each other and exclude the fats

• In a fat, three fatty acids are joined to glycerol by an ester linkage, creating a triacylglycerol, or triglyceride

Ester linkage

Fat molecule (triacylglycerol)

Glycerides

• Glycerol + 1 fatty acid = monoglyceride Glycerol + 1 fatty acid = monoglyceride

• Glycerol + 2 fatty acids = diglyceride Glycerol + 2 fatty acids = diglyceride

• Glycerol + 3 fatty acids = triacylglycerol Glycerol + 3 fatty acids = triacylglycerol (also called a triglyceride or fat.)(also called a triglyceride or fat.)

Saturated Fats• Fats made from saturated fatty acids are called saturated

fats

• Most animal fats are saturated

• Saturated fats are solid at room temperature

• A diet rich in saturated fats may contribute to cardiovascular disease through plaque deposits

Saturated fat and fatty acid.

Stearic acid

Unsaturated Fats• Fats made from unsaturated fatty acids are called

unsaturated fats

• Plant fats and fish fats are usually unsaturated

• Plant fats and fish fats are liquid at room temperature and are called oils

Unsaturated fat and fatty acid.

Oleic acid

cis double bondcauses bending

Fat Sources

• Most animal fats contain saturated fatty acids and tend to be solid at room temperature

• Most plant fats contain unsaturated fatty acids. They tend to be liquid at room temperature, and are called oils.

Phospholipids• In phospholipids, two of the –OH groups on glycerol are joined to

fatty acids. The third –OH joins to a phosphate group which joins, in turn, to another polar group of atoms.

• The phosphate and polar groups are hydrophilic (polar head) while the hydrocarbon chains of the 2 fatty acids are hydrophobic (nonpolar tails).

Structural formula Space-filling model Phospholipid symbol

Hydrophilichead

Hydrophobictails

Fatty acids

Choline

Phosphate

Glycerol

Hyd

rop

ho

bic

tai

lsH

ydro

ph

i lic

hea

d

Micelle

Phospholipid bilayer Water

Water

Water Lipid head (hydrophilic)

Lipid tail (hydrophobic)

Phospholipids• When phospholipids are added to water, they orient so that the

nonpolar tails are shielded from contact with the polar H2O may form micelles

• Phosopholipids also may self-assemble into a bilayer, with the hydrophobic tails pointing toward the interior

Phospholipids• The structure of phospholipids results in a bilayer arrangement

found in cell membranes

• Phospholipids are the major component of all cell membranes

WATERHydrophilichead

Hydrophobictails

WATER

Steroids• Steroids are lipids characterized by a carbon

skeleton consisting of four fused rings• Cholesterol, an important steroid, is a component

in animal cell membranes• Testosterone and estrogen function as sex

hormones

Proteins• Proteins have many structures, resulting in a wide

range of functions• They account for more than 50% of the dry mass

of most cells• Protein functions

– Structural support / storage / movement - fibers – Catalysis - Enzymes– Defense against foreign substances– Immunoglobulins– Transport – globins, membrane transporters– Messengers for cellular communications - hormones

Proteins• A protein is composed of one or more polypeptides that

performs a function• A polypeptide is a polymer of amino acids joined by

peptide bonds to form a long chain• Polypeptides range in length from a few monomers to

more than a thousand• Each polypeptide has a unique linear sequence of amino

acids• A protein consists of one or more polypeptides which are

coiled and folded into a specific 3-D shape. • The shape of a protein determines its function.

Amino Acids• Amino acids are monomers of polypetides

• They composed of a carboxyl group, amino group, and an “R”Group

• Amino acids differ in their properties due to differing side chains, called R groups

• Cells use 20 amino acids to make thousands of proteins

Aminogroup

Carboxylgroup

carbon

O

O–

H

H3N+ C C

O

O–

H

CH3

H3N+ C

H

C

O

O–

CH3 CH3

CH3

C C

O

O–

H

H3N+

CH

CH3

CH2

C

H

H3N+

CH3

CH3

CH2

CH

C

H

H3N+ C

CH3

CH2

CH2

CH3N+

H

C

O

O–

CH2

CH3N+

H

C

O

O–

CH2

NH

H

C

O

O–

H3N+ C

CH2

H2C

H2N C

CH2

H

C

Nonpolar

Glycine (Gly) Alanine (Ala) Valine (Val) Leucine (Leu) Isoleucine (Ile)

Methionine (Met) Phenylalanine (Phe)

C

O

O–

Tryptophan (Trp) Proline (Pro)

H3C

Figure 5.17

S

O

O–

Amino Acids

O–

OH

CH2

C C

H

H3N+

O

O–

H3N+

OH CH3

CH

C C

HO–

O

SH

CH2

C

H

H3N+ C

O

O–

H3N+ C C

CH2

OH

H H H

H3N+

NH2

CH2

O

C

C C

O

O–

NH2 O

C

CH2

CH2

C CH3N+

O

O–

O

Polar

Electricallycharged

–O O

C

CH2

C CH3N+

H

O

O–

O– O

C

CH2

C CH3N+

H

O

O–

CH2

CH2

CH2

CH2

NH3+

CH2

C CH3N+

H

O

O–

NH2

C NH2+

CH2

CH2

CH2

C CH3N+

H

O

O–

CH2

NH+

NH

CH2

C CH3N+

H

O

O–

Serine (Ser) Threonine (Thr)Cysteine

(Cys)Tyrosine

(Tyr)Asparagine

(Asn)Glutamine

(Gln)

Acidic Basic

Aspartic acid (Asp)

Glutamic acid (Glu)

Lysine (Lys) Arginine (Arg) Histidine (His)

Amino Acids

Amino Acids and Peptide Bonds

• Two amino acids can join by condensation to form a dipeptide plus H2O.

• The bond between 2 amino acids is called a peptide bond.

Protein Conformation and Function• A functional protein consists

of one or more polypeptides twisted, folded, and coiled into a unique shape

• The sequence of amino acids determines a protein’s three-dimensional conformation

• A protein’s conformation determines its function

• Ribbon models and space-filling models can depict a protein’s conformation

A ribbon model

Groove

Groove

A space-filling model

Four Levels of Protein Structure• The primary structure of a protein is its unique sequence of amino

acids

• Secondary structure, found in most proteins, consists of coils and folds in the polypeptide chain

• Tertiary structure is determined by interactions among various side chains (R groups)

• Quaternary structure results when a protein consists of multiple polypeptide chains

Amino acidsubunits

pleated sheet

helix

Primary Structure

• Primary structure, the sequence of amino acids in a protein, is like the order of letters in a long word

• Primary structure is determined by inherited genetic information

Amino acidsubunits

Carboxyl end

Amino end

Secondary Structure• The coils and folds of secondary structure result from

hydrogen bonds between repeating constituents of the polypeptide backbone

• Typical secondary structures are a coil called an alpha helix and a folded structure called a beta pleated sheet

Amino acidsubunits

pleated sheet

helix

Tertiary Structure• Tertiary structure is

determined by interactions between R groups, rather than interactions between backbone constituents

• These interactions between R groups include hydrogen bonds, ionic bonds, hydrophobic interactions, and van der Waals interactions

• Strong covalent bonds called disulfide bridges may reinforce the protein’s conformation

Hydrophobicinteractions andvan der Waalsinteractions

Polypeptidebackbone

Disulfide bridge

Ionic bond

Hydrogenbond

Quaternary Structure• Quaternary structure results when two or more polypeptide

chains form one macromolecule• Collagen is a fibrous protein consisting of three

polypeptides coiled like a rope• Hemoglobin is a globular protein consisting of four

polypeptides: two alpha and two beta chains

Chains

ChainsHemoglobin

Iron

Heme

CollagenPolypeptide chain

Polypeptidechain

Levels of Protein Structure

68

Interactions that Contribute to a Interactions that Contribute to a Protein’s ShapeProtein’s Shape

68 68 68

69

Sickle-Cell Disease: A Simple Change in Primary Structure

• A slight change in primary structure can affect a protein’s conformation and ability to function

• Sickle-cell disease, an inherited blood disorder, results from a single amino acid substitution in the protein hemoglobin

Red bloodcell shape

Normal cells arefull of individualhemoglobinmolecules, eachcarrying oxygen.

10 µm 10 µm

Red bloodcell shape

Fibers of abnormalhemoglobin deformcell into sickleshape.

Sickle Cell Anemia

Primarystructure

Secondaryand tertiarystructures

1 2 3

Normal hemoglobin

Val His Leu

4Thr

5Pro

6Glu Glu

7Primarystructure

Secondaryand tertiarystructures

1 2 3

Sickle-cell hemoglobin

Val His Leu

4Thr

5Pro

6Val Glu

7

Quaternarystructure

Normalhemoglobin(top view)

Function Molecules donot associatewith oneanother; eachcarries oxygen.

Quaternarystructure

Sickle-cellhemoglobin

Function Molecules interact withone another tocrystallize intoa fiber; capacityto carry oxygenis greatly reduced.

Exposedhydrophobicregion subunit subunit

Conformation• In addition to primary structure, physical and chemical conditions can

affect conformation

• Alternations in pH, salt concentration, temperature, or other environmental factors can cause a protein to unravel

• This loss of a protein’s native conformation is called denaturation

• A denatured protein is biologically inactive

Denaturation

Renaturation

Denatured proteinNormal protein

Nucleic Acids

• Nucleic acids store and transmit hereditary information

• The amino acid sequence of a polypeptide is programmed by a unit of inheritance called a gene

• Genes are made of DNA, a nucleic acid

The Roles of Nucleic Acids

• There are two types of nucleic acids:– Deoxyribonucleic acid (DNA)– Ribonucleic acid (RNA)

• DNA provides directions for its own replication• DNA directs synthesis of messenger RNA

(mRNA) and, through mRNA, controls protein synthesis

• Protein synthesis occurs in ribosomes

NUCLEUS

DNA

CYTOPLASM

mRNA

mRNA

Ribosome

Aminoacids

Synthesis ofmRNA in the nucleus

Movement ofmRNA into cytoplasmvia nuclear pore

Synthesis of protein

Polypeptide

The Structure of Nucleic Acids

• Nucleic acids are polymers called polynucleotides• Each polynucleotide is made of monomers called

nucleotides• Each nucleotide consists of a nitrogenous base, a

pentose sugar, and a phosphate group• The portion of a nucleotide without the phosphate

group is called a nucleoside

5 end

3 end

Nucleoside

Nitrogenousbase

Phosphategroup

Nucleotide

Polynucleotide, ornucleic acid

Pentosesugar

Phosphate group

Sugar

Nitrogenous base

N

N

O

4’

5’

1’

3’ 2’

2 8

7 6

3 9 4

5

P CH2

O

– O

O –

OH R OH in RNA

H in DNA

O

N

NH2

N 1

Nucleotide Monomers• Nucleotide monomers are made

up of nucleosides and phosphate groups

• Nucleoside = nitrogenous base + sugar

• There are two families of nitrogenous bases: – Pyrimidines have a single six-

membered ring– Purines have a six-membered ring

fused to a five-membered ring

• In DNA, the sugar is deoxyribose• In RNA, the sugar is ribose

Nitrogenous bases

Pyrimidines

Purines

Pentose sugars

CytosineC

Thymine (in DNA)T

Uracil (in RNA)U

AdenineA

GuanineG

Deoxyribose (in DNA)

Nucleoside components

Ribose (in RNA)

Nitrogenous bases

• Purines have a double ring structure and include adenine (A) and guanine (G).

• Pyrimidines have a single ring structure and include cytosine (C), thymine (T), and uracil (U) – found only in RNA

Adenine

Guanine

C C

N N

N

C

H

N

C

C H

O

H

Cytosine (both DNA and RNA)

Thymine (DNA only)

Uracil (RNA only)

H C C

N C

H

N

C

NH2

N

N

C H O C C

N C

H

N

C H

H

O C C

N C

H

N

C

O

H H3C

H

O C C

N C

H

N

C

O

H H

H

P U R I N E S

P Y R I M I D I N E S

NH2

NH2

Nucleotide Polymers• Nucleotide polymers are linked

together, building a polynucleotide• Adjacent nucleotides are joined by

covalent bonds that form between the –OH group on the 3´ carbon of one nucleotide and the phosphate on the 5´ carbon on the next

• These links create a backbone of sugar-phosphate units with nitrogenous bases as appendages

• The sequence of bases along a DNA or mRNA polymer is unique for each gene

The DNA Double Helix• A DNA molecule has two

polynucleotides spiraling around an imaginary axis, forming a double helix

• In the DNA double helix, the two backbones run in opposite 5´ to 3´ directions from each other, an arrangement referred to as antiparallel

• One DNA molecule includes many genes

• The nitrogenous bases in DNA form hydrogen bonds in a complementary fashion: A always with T, and G always with C

Sugar-phosphatebackbone

3 end5 end

Base pair (joined byhydrogen bonding)

Old strands

Nucleotideabout to beadded to anew strand

5 end

New strands

3 end

5 end3 end

5 end

ATP• Adenosine triphosphate (ATP), is the primary energy-

transferring molecule in the cell • ATP is the “energy currency” of the cell• ATP consists of an organic molecule called adenosine

attached to a string of three phosphate groups• The energy stored in the bond that connects the third

phosphate to the rest of the molecule supplies the energy needed for most cell activities